A powerful anti-illegal-guns group has attracted more than 400,000 new members since the Dec. 14 shooting in Newtown, Conn., a spokeswoman told POLITICO Thursday, swinging back several hours after the National Rifle Association also claimed big membership gains.

“Since Newtown we’ve had more than 400k people join Mayors Against Illegal Guns,” said Erika Soto Lamb, the communications director for the organization, in an email. “Our Demand A Plan petition now has more than 900,000 petition signers.”

Mayors Against Illegal Guns, closely tied to New York City Mayor Michael Bloomberg, is a coalition that includes more than 800 mayors opposed to illegal gun use: “what binds us together is a determination to fight crime, and a belief that we can do more to stop criminals from getting guns while also protecting the rights of citizens to freely own them,” the organization says.

Earlier Thursday, the NRA told POLITICO that the organization gained 100,000 new members over the last 18 days, bringing its total number of paid members from 4.1 million to 4.2 million.

Mayors Against Illegal Guns launched its “Demand A Plan” initiative in the wake of last summer’s shootings at a movie theater in Aurora, Colo. The campaign, which involves a petition as well as several ad buys, calls on President Barack Obama and on Congress to “step forward with a plan to end gun violence.”